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Title: Developing acoustic metamaterial with forward and inverse design methodology
Authors: Gao, He
Degree: Ph.D.
Issue Date: 2021
Abstract: Acoustic metamaterials are engineered materials that promise properties otherwise hard or impossible to find in nature. They offer more possibilities to manipulate sound waves and open the door for improved or completely new applications. This thesis has examined two different approaches for acoustic metamaterials design: the forward design and inverse design. The forward design methodology is a straightforward and widely used strategy, which can be utilized to design the metamaterial devices with analytically predicted performance. But for complex environment or unstable systems, the analytical solutions are not always there to be obtained. For those cases, the inverse design approach will play a much more significant role in acoustic metamaterial development. This thesis starts by designing an acoustic metamaterial lens in air with forward design method. In the designed lens, the waveguiding effect is explored, which could guide the Airy beam to propagate along a sine-like path. At the same time, one-dimensional Talbot effect is demonstrated, in which the pattern of the incident field can be reproduced at some special positions. This multifunctional metamaterial lens successfully reunifies the geometric approach with wave acoustics to support simultaneous sound guiding and Talbot effect, which can be experimentally observed in a relatively broad frequency range. This thesis further extends the one-dimensional Talbot effect to two-dimension, which could be controlled from two wave dimensions and diverse imaging patterns have been achieved with coding metasurface. By altering the coding sequences in the metamaterial lens, the imaging period and intensity can be flexibly controlled.
Also, with forward design method, the following chapter of this thesis further demonstrates how to design the newly developed metamaterials that allow for topologically protected edge and corner states, distinct from the above mentioned conventional metamaterials. The one-dimensional and two-dimensional topological insulators consisting of coupled resonators are respectively fabricated in acoustic metamaterial lattices. Their topological properties are induced by the non-Hermiticity, which is controlled by introducing sound leakage in certain sites in the lattices. Consequently, the topologically protected in-gap edge states (in one-dimensional lattice) and corner states (in two-dimensional lattice) can be observed, in which the energy localization can be clearly observed. Moreover, the concept of acoustic metamaterial is not limited in air, rather showing enormous potential applications in water, where the inverse design strategy is a more appropriate and powerful tool. Thus, the following chapter of the thesis shows how to optimize the metamaterial underwater devices with the inverse design method, which are challenging to be realized with forward approach. The acoustic invisible devices with different shapes are first demonstrated, which possess the simplest structure and consist of commonly used materials. This concept of inverse design is further utilized to optimize the metamaterial devices that can enhance the acoustic power transmission through the hard plate immersed in water. Once the plate is patterned with the optimized T-shaped grating, the transmitted sound energy can be remarkably enhanced and focused. This design approach offers the possibility to design acoustic metamaterial devices with arbitrary shapes under various circumstances.
Subjects: Metamaterials -- Acoustic properties
Metamaterial -- Design and construction
Hong Kong Polytechnic University -- Dissertations
Pages: ix, 117 pages : color illustrations
Appears in Collections:Thesis

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